Nearly 90% of trauma-exposed Veterans report clinically significant insomnia and/or trauma-related nightmares. Added to the impact on posttraumatic stress disorder (PTSD), trauma-related nightmares have a negative effect on physical health, substance use, and suicidal behaviors. There is a critical need for continued research to better understand these phenomena with the goal of developing personalized treatment plans. Limitations of current assessment procedures present a significant barrier to improved care. In-laboratory sleep studies rarely capture nightmares and cannot assess change over time, reducing our knowledge of phenotypic markers of nightmares to guide treatment. Therefore, there is a need to assess sleep over multiple nights in the home environment, where nightmares occur. The proposed Clinical Sciences Research and Development CDA-2 aims to significantly enrich knowledge about trauma-related nightmares by using a zero-burden, multi- night, objective sleep measurement method within Veterans? usual sleeping environment prior to and during an evidence-based cognitive-behavioral intervention for nightmares. This study has two primary aims: 1) to identify, with greater precision than previously possible, objective features of sleep associated with trauma- related nightmare occurrences; and 2) to use the treatment for nightmares as interventional probes to determine whether and how changes in sleep physiological parameters identified in Aim 1 covary with changes in subjective nightmare frequency and severity. The study will include 80 trauma-exposed Veterans reporting with trauma-related nightmares. Eligible participants will monitor their sleep for a week using a multi-night mattress actigraphy implemented in their home. Mattress actigraphy, which measures movements using accelerometers embedded in a mattress topper, employs no body surface sensors. Therefore, this system represents a truly zero-burden method for obtaining intensive longitudinal sleep measurement. During the week of sleep monitoring, participants also will complete one-night of polysomnography (PSG) sleep assessment to calibrate the actigraphic sleep efficiency. These methods will be used to investigate candidate physiological parameters associated with trauma-related nightmares. After establishing the levels of these candidate markers, this project will assess the impact of a cognitive-behavioral treatment, Exposure, Relaxation, and Rescripting Therapy (ERRT), on the subset of markers which can be measured continuously over the course of the treatment. Participants will be randomized to five weeks of active treatment (ERRT; n = 40) or to five weeks of the comparison treatment (sleep and nightmare management; n = 40). Throughout the course of treatment, participants will continue to sleep while monitored by the mattress actigraphy system. Upon completion of treatment, a post-treatment and follow-up assessment will assess subjective symptom change. Results from this CDA-2 will provide important information to facilitate increased understanding of the phenomenology, pathophysiology, and treatment of nightmares in trauma-exposed Veterans. This CDA-2 also will provide the applicant with focused training in sleep physiology, advanced longitudinal statistical designs and treatment research methods, and grantsmanship to prepare for a successful VA research career focused on improving the quality of life for Veterans with trauma-related sleep disturbances.